What is a mirror, truly?
even if it reflects better than a mirror, it will never be one.
While Deconstructionism is a tricky and oft-maligned practice, it is better than too firm a grip on terms and words. The 'true mirror' mirror you speak of can hold crackers, and cheese. One could drive nails into the wall if they wanted to, if they logically perceived the possibility.
The idea of 'polishing the mirror' as I understand it is quite common to development of skill and understanding of the martial arts / budo in general. It implies that something is not what it will become. There is a different manifestation of aikido for every body and mind type on the mat. Some individuals spring to it like fish to water, both mentally and physically, they are good mirrors. Others struggle to make it to and through class, and are baffled by some of the stuff that only baffles most of us slightly less so. These are bad mirrors. It is best to be a bad mirror. The effort put forth is that much more precious. If it is impossible to reach a goal or state that is other than our present selves, then we should head to the bar, as we've reached nirvana and never yet noticed.
Juan did not claim the need to hold shinto beliefs, he inquired as to the need, as it was not in his heart as you rightly stated, and worried that he might be wasting his time in aikido if he ultimately needed to believe in shinto spirituality. But understanding the motivations behind the message will communicate knowledge, without demanding belief in the messenger. Striving outright for ignorance because one encounters something they don't want to believe is like putting the mirror in a sack, and putting the polishing tools back on the shelf.
I think I'm getting too far off track, so I will leave it at that. I'm probably just thinking too much on a thursday. So, forgive me for waxing verbose.